1. Field of the Invention
The present invention relates to a quadrature demodulator that creates an I-signal and a Q-signal of a baseband to demodulate a reception signal.
2. Description of the Related Art
A radio-frequency identification (RFID) communication device makes radio communication with a transponder called an RFID tag. The RFID communication device transmits data to the RFID tag using a modulated radio wave, and after the end of the data transmission, transmits a continuous wave (CW). In contrast, the RFID tag carries out backscatter modulation by changing a reflection amount of the continuous wave from the RFID communication device, to transmit data to the RFID communication device. The RFID communication device receives a backscatter modulation wave and reads data on the RFID tag.
The RFID communication device comprises a transmitting section and a receiving section. In the transmitting section, a modulator modulates data, and an amplifier amplifies the data to be transmitted to the RFID communication device through an antenna. In the receiving section, a direct conversion receiver extracts a baseband signal from a high-frequency signal that is the signal received by an antenna, and then demodulates the baseband signal to derive data.
When a quadrature demodulator is provided for direct conversion, the reception signal and a local signal having a frequency that is equal to that of a carrier of the reception signal are input to a mixer to create an in-phase (I) signal of a baseband, and the reception signal and a signal shifted in phase by 90 degrees with respect to the local signal are input to a mixer to create a quadrature-phase (Q) signal of a baseband.
The amplitudes of the I- and Q-signals depend on a phase difference between the reception signal and the local signal. The maximum amplitude of the I-signal minimizes the amplitude of the Q-signal. The minimal amplitude of the I-signal maximizes the amplitude of the Q-signal. When the Q-signal has a minimal value of 0, the I-signal has the maximum amplitude. Thus, the use of the I-signal enables reproduction of data received. In contrast, when the I-signal has a minimal value of 0, the Q-signal has the maximum amplitude. Thus, the use of the Q-signal enables reproduction of data received. Further, there is a case in which the phases of the I- and Q-signals are inverted depending on the phase difference between the reception signal and the local signal.
A known reception data reproduction method that uses the direct conversion quadrature demodulator compares the amplitude of the I-signal with that of the Q-signal to select the signal having a greater amplitude, and then reproduces reception data (see, for example, U.S. Pat. No. 6,501,807 B1).
In the method described in U.S. Pat. No. 6,501,807 B1, the signal having a greater amplitude is selected to reproduce the reception data as a result of comparison between the amplitudes of the I-signal and the Q-signal. Accordingly, when the amplitude of the I-signal significantly differs from that of the Q-signal, a sufficient amplitude that enables reproduction without any difficulty is obtained in the selected signal.
However, when the amplitude of the I-signal is substantially equal to that of the Q-signal, either of the signals may be selected, but reproduction of reception data must be conducted at an amplitude that is half of that of the reception signal. Thus, disadvantageously, the reception signal at a low level is likely to suffer from noise, resulting in the frequent noise-induced incorrect reproduction of the reception data.